Intermittent fasting has become an increasingly popular dietary strategy in recent years. But is restricting your eating window down to just a few hours a day really the healthiest way to eat? Here is a comprehensive look at the evidence behind intermittent fasting.
What is intermittent fasting?
Intermittent fasting (IF) is an eating pattern that involves cycling between periods of fasting and eating. It doesn’t specify which foods you should eat, but rather when you should eat them. There are several different IF protocols, including:
- 16/8 method: Fast for 16 hours per day, restrict eating to an 8-hour window.
- 5:2 diet: Eat normally 5 days a week, restrict calories to 500–600 per day the other 2 days.
- Eat-stop-eat: Fast for 24 hours once or twice per week, eating normally the other days.
- Alternate day fasting: Fast every other day, eating freely on non-fasting days.
The most common IF protocol is the 16/8 method, where you skip breakfast and restrict eating to the afternoon and evening. The 5:2 diet is also popular, where you eat very little just two days per week.
Purported benefits of intermittent fasting
Proponents claim that intermittent fasting can provide these benefits:
- Weight loss: By limiting when you eat, you reduce your calorie intake and burn stored body fat.
- Insulin sensitivity: Fasting may improve your body’s response to insulin, lowering blood sugar.
- Longer lifespan: Animal studies link IF with increased longevity and resistance to age-related diseases.
- Cellular repair: Periods of fasting may induce processes that remove damaged cells and recycle parts.
- Disease prevention: Intermittent fasting may help prevent chronic diseases like diabetes, cancer and heart disease.
- Cognitive function: Fasting may improve memory, learning, problem-solving and focus.
However, most of these purported benefits come from animal studies. Human research on intermittent fasting is still emerging.
Weight loss effects
Weight loss is one of the most commonly promoted benefits of intermittent fasting. By only eating during a specific window each day, you reduce your overall calorie intake without actively trying to restrict calories.
Studies show that intermittent fasting can be an effective weight loss strategy if it leads to an overall reduction in calories. However, the effect seems equivalent to typical calorie restriction (1, 2).
In a review study, people lost an average of 7–11 pounds (3–5 kg) over 10 weeks with various intermittent fasting approaches. However, they only lost 0.25–1 pound (0.1–0.5 kg) more on average than the control groups (3).
When comparing intermittent fasting to continuous calorie restriction, most studies show no difference in weight loss if calories are matched between groups (4).
That said, some studies note modestly greater weight loss with intermittent fasting in the short-term. This may be due to slight spontaneous reductions in calorie intake (5).
One problem is that the long-term effects of intermittent fasting on weight beyond 1 year haven’t been researched much. However, there is some evidence suggesting intermittent fasting may not lead to greater weight loss or fat loss in the long-term (6).
Overall, intermittent fasting can be an effective weight loss strategy if it leads you to eat fewer calories overall. But it does not seem to have any special effects on metabolism or fat loss beyond that.
Effects on metabolism and hormones
Intermittent fasting is sometimes claimed to boost metabolism by increasing fat burning and preserving muscle mass. However, the effects on metabolism are mixed.
Some studies show that fasting increases fat burning and metabolism more than calorie restriction (7, 8). However, several larger studies found no difference in metabolism between intermittent fasting and continuous calorie restriction (9, 10).
Studies also reveal that IF does not provide superior muscle preservation compared to traditional calorie-matched weight loss (11).
Intermittent fasting seems to provide a minor boost to metabolism, at best. However, the effects diminish as you become adapted to the fasting routine.
As far as hormones, short-term fasts generally increase the levels of human growth hormone (HGH) by up to 5-fold. Higher levels help burn fat and preserve muscle mass (12, 13).
However, fasting periods longer than 24 hours can suppress the production of thyroid hormones T3 and T4, which play an important role in metabolism (14).
Overall, research does not support fasting providing a significant long-term boost to your metabolism. The hormonal effects may provide small, short-term benefits.
Effects on blood sugar and insulin sensitivity
Several studies have found intermittent fasting to have major benefits for insulin resistance and blood sugar levels.
In one study, fasting helped reduce insulin resistance by up to 75% (15).
Large observational studies also show reductions in diabetes risk among those who fast periodically (16, 17).
However, these results are controversial. Other studies found no effects, and even that fasting increased insulin levels and insulin resistance in some cases (18).
Several factors may play a role here, including the length of the fasting period and the health status of participants prior to starting.
But there is some evidence that intermittent fasting can temporarily improve blood sugar control and insulin sensitivity, at least in the short-term.
Anti-aging and longevity
Evidence from animal models indicates that intermittent fasting may extend lifespan by slowing the aging process and protecting against disease (19).
Studies in rats and mice found that intermittent fasting extends lifespan by up to 80% in some cases (20). Fasting appears to trigger adaptive cellular stress responses that strengthen cells against later oxidative stress (21).
Additionally, fasting promotes autophagy, a cellular “housekeeping” process where damaged parts of cells are recycled (22).
This process may prevent aging-related diseases, including neurodegenerative conditions like Alzheimer’s disease (23).
However, overall evidence in humans is limited. Only two studies directly compared intermittent fasting against continuous calorie restriction for anti-aging effects (24, 25).
Neither found a difference in health markers after 1–3 years of alternate-day fasting or twice-weekly fasting.
Animal research indicates powerful anti-aging effects from fasting. But the limited human data shows no difference between intermittent fasting and continuous calorie restriction.
Disease prevention
Animal research indicates that intermittent fasting may prevent chronic disease, including cancer and heart disease (26, 27).
However, the human research is still emerging.
One small study found that 4 weeks of alternate-day fasting reduced risk factors for heart disease, including blood pressure, cholesterol and triglycerides (28).
Larger, long-term studies also observed reductions in heart disease risk factors, although they depended on substantial calorie restriction as well (29, 30).
Additionally, some observational studies suggest a link between fasting and a lower rate of cancer. However, more controlled studies are needed (31).
Overall, evidence is limited but indicates that intermittent fasting done long-term may help reduce some heart disease risk factors. Cancer prevention effects are possible but unproven.
Cognitive function
A handful of studies have linked intermittent fasting with improved cognitive function, including memory, learning, problem-solving and focus (32, 33).
Part of this may stem from fasting’s anti-aging effects and reductions in inflammation. However, research directly testing effects on brain function is still new.
Early findings are promising, but more research is needed on intermittent fasting’s direct effects on age-related cognitive decline.
Downsides to consider
Despite the health benefits of intermittent fasting, it does come with some downsides to consider.
For starters, fasting is difficult for many people, especially at first. Hunger, irritability and reduced ability to concentrate are common complaints.
Intermittent fasting regimens also restrict when you can eat, which can make it hard to plan meals and social gatherings. Following very low-calorie fast days requires significant meal planning to meet nutritional needs.
Another issue is that some people report overeating or binge eating during non-fasting periods, potentially negating some of the benefits. More research is needed on the risks of weight cycling and metabolic adaptations from intermittent fasting (34).
Intermittent fasting is also not recommended for certain groups, including children/teens, pregnant or breastfeeding women, older adults, and those who are underweight or have a history of eating disorders.
Lastly, fasting may worsen gut issues like acid reflux or heartburn. For this reason, it may be best for some people to start slow or modify fasting protocols to lessen symptoms.
The evidence behind “time-restricted eating”
“Time-restricted eating” is a form of daily intermittent fasting where meal timing is limited to set hours. The most common approach is fasting for 16–20 hours per day and restricting eating to 4–8 hours.
Studies on this type of daily fasting show it can help people lose weight and body fat, especially when combined with resistance training (35, 36).
Research also shows that time-restricted eating can reduce core body temperature and improve sleep quality in men with prediabetes (37).
However, these were all small, short-term studies. Larger and longer-term studies are needed, especially on women and older adults.
Overall, time-restricted eating shows promise as a weight loss strategy. But more research is needed on its long-term efficacy and safety.
Intermittent fasting during training
Athletes and highly active individuals commonly do short fasts before exercise to reach lower body fat percentages. This strategy, called “fasted training,” may help boost fat burning during exercise (38).
However, research indicates no significant improvements in endurance capacity or performance with fasted training (39).
Plus, practicing fasted exercise regularly can negatively impact female reproductive health and bone mineral density over time. Most research has focused on men (40).
At this point, not enough evidence supports intermittent fasting for athletes. More research is underway on how nutritional timing affects health and performance.
Different types of fasting
Here is an overview of the most common intermittent fasting and nutritional cycling protocols:
| Type | Fasting Schedule |
|---|---|
| 16/8 method | 16 hours fasting, 8-hour eating window daily |
| 5:2 diet | Normal eating 5 days a week, 500 calories 2 days a week |
| Eat-stop-eat | 24-hour fast 1–2 times per week on non-consecutive days |
| Alternate day fasting | Alternate between fasting and feasting days |
| Warrior diet | 20-hour fast, 4-hour eating window daily |
| 14/10 method | 14-hour fasts, 10-hour eating window daily |
Tips to start intermittent fasting
Here are some tips to get started with intermittent fasting:
- Start slow: Try doing 12-hour fasts for a few days to get used to it before increasing to a 16-hour fast.
- Drink water: Stay hydrated, especially during the fasting period. Coffee, tea and other non-caloric beverages are fine too.
- Plan your meals: Make sure to eat enough during your eating window, choosing nutrient-dense foods.
- Listen to your body: Modify or stop fasting if you feel ill or have persistent hunger, fatigue or other negative symptoms.
- Exercise: Light exercise on fast days can help boost fat burning.
- Supplement if needed: Take electrolytes before longer fasts and eat enough protein on feeding days.
It may take some experimentation to find the right intermittent fasting protocol that fits your lifestyle and doesn’t cause negative symptoms.
Does intermittent fasting work long-term?
Research shows that intermittent fasting can produce impressive short-term weight loss and health benefits. However, studies longer than 1 year are still limited (6).
Reports from medical practitioners and testimony from those who have practiced intermittent fasting for years suggest that many people find it a sustainable lifestyle.
That said, some individuals find complying with fasting regimens too difficult long-term. More research is needed on compliance rates and the effects intermittent fasting has over several years.
Should you try intermittent fasting?
Intermittent fasting can be a successful weight loss strategy if it leads you to eat less overall. Many people find short-term fasting relatively easy to comply with.
Animal research indicates that intermittent fasting may provide far-reaching health benefits, from slowing aging to preventing chronic disease. However, most human research has focused on short-term outcomes.
At this time, intermittent fasting has a solid foundation of evidence demonstrating weight loss and short-term health effects in humans. However, more high quality research is needed on long-term efficacy and safety.
Overall, intermittent fasting appears to be a potentially promising health strategy for those who can comply with the dietary pattern long-term.
Conclusion
Research to date suggests intermittent fasting can produce short-term weight loss similar to traditional calorie restriction. However, there is currently insufficient evidence that intermittent fasting provides significant long-term metabolic or weight loss benefits beyond those seen with continuous calorie restriction.
More high quality, large scale studies lasting 1 year or longer are needed to definitively determine whether intermittent fasting is effective and safe in the long-term. Several potentially beneficial effects on aging, metabolism, and disease prevention seen in animal studies also require validation through rigorous controlled trials in humans.
For most healthy active adults, intermittent fasting is likely a safe dietary strategy for fat loss and metabolic health if it can be maintained long-term. However, more research is needed on effects in women, older adults and other populations.
Overall, intermittent fasting requires more research before strong conclusions can be made on its long-term efficacy. While the practice shows early promise, adopting intermittent fasting should be done with caution until larger and longer-term studies are completed.
